The present invention relates to coating particles and fibers, particularly to coating powders and fibers to modify the physical and chemical properties thereof, and more particularly to a process and apparatus for producing adherent coatings on very small particles, powders, and fibers by magnetron sputtering two layers thereon without interface contamination.
Coatings make it possible to modify the physical and chemical properties of powders and small fibers. Coatings provide a method of improving the compatibility of powders, for example, with other materials and in certain processes. Diamond powder, for example, is coated with nickel, for example, to improve its electrical conductivity. Coated diamond is also used in composite grinding tools because it bonds better to most matrix materials than bare diamond. Diamond is one of only a few materials that are available as a coated powder. These coatings are generally prepared by electrochemical plating processes. Although coating uniformity is very good, coating adhesion is only marginal. The known commercial coating processes appear to have problems with flexibility, adhesion, or coverage. Further, there are no known coating processes that are capable of applying uniform and adherent coatings to small powders and fibers in the micron and submicron size. The potential benefit of a coated powder in most applications is determined by the quality of the coating, which includes its adhesive capability. Consequently, the benefits of using coated powders in many material processes has not been evaluated.
The above-referenced coating problems have been overcome by the coating process and apparatus of the present invention. The process of this invention has made it possible to apply an adherent and uniform coating to each particle in a powder sample, for example. Particles with diameters as small as one micron have been successfully coated by this process, which involves a two layer coating without layer interface contamination. The exceptional quality of the coating deposited on small particles and fibers by the process of this invention could lead to the development of improved composite materials and new fabrication processes. The process is carried out using magnetron sputtering of a first thin adhesion layer followed without interruption by at least a second thicker layer of desired material. Prior to coating, the particles and fibers are cleaned by a plasma cleaning operation. Uniformity of coating is maintained by agitation of the particles using the apparatus of the invention.